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How to Think Like a Computer Scientist

Chapter 20

Trees

Like linked lists, trees are made up of nodes. A common kind of tree
is a binary tree, in which each node contains a reference to two
other nodes (possibly null). These references are referred to as the
left and right subtrees. Like list nodes, tree nodes also contain
cargo. A state diagram for a tree looks like this:

To avoid cluttering up the picture, we often omit the
Nones.

The top of the tree (the node tree refers to) is called the
root. In keeping with the tree metaphor, the other nodes are
called branches and the nodes at the tips with null references are
called leaves. It may seem odd that we draw the picture with
the root at the top and the leaves at the bottom, but that is not the
strangest thing.

To make things worse, computer scientists mix in another
metaphor the family tree. The top node is sometimes called
a parent and the nodes it refers to are its children.
Nodes with the same parent are called siblings.

Finally, there is a geometric vocabulary for talking
about trees. We already mentioned left and right, but there is
also "up" (toward the parent/root) and "down" (toward the
children/leaves). Also, all of the nodes that are the same
distance from the root comprise a level of the tree.

We probably don't need three metaphors for talking about trees,
but there they are.

Like linked lists, trees are recursive data structures because
they are defined recursively.

A tree is either:

the empty tree, represented by None, or

a node that contains an object reference and two
tree references.

20.1 Building trees

The process of assembling a tree is similar
to the process of assembling a linked list.
Each constructor invocation builds a single node.

The cargo can
be any type, but the arguments for left and right should be
tree nodes. left and right are optional; the default
value is None.

To print a node, we just print the cargo.

One way to build a tree is from the bottom up.
Allocate the child nodes first:

left = Tree(2)
right = Tree(3)

Then create the parent node and link it to the children:

tree = Tree(1, left, right);

We can write this code more concisely by nesting constructor
invocations:

>>> tree = Tree(1, Tree(2), Tree(3))

Either way, the result is the tree at the beginning of the
chapter.

20.2 Traversing trees

Any time you see a new data structure, your first
question should be, "How do I traverse it?" The most natural
way to traverse a tree is recursively. For example, if the
tree contains integers as cargo, this function returns their sum:

The base case is the empty tree, which contains no cargo, so
the sum is 0.
The recursive step
makes two recursive calls to find the sum of the child trees.
When the recursive calls complete,
we add the cargo of the parent and return the
total.

20.3 Expression trees

A tree is a natural way to represent the structure of an expression.
Unlike other notations, it can represent the computation
unambiguously. For example, the infix expression 1 + 2 * 3 is
ambiguous unless we know that the multiplication happens before the
addition.

This expression tree represents the same computation:

The nodes of an expression tree can be operands like 1 and
2 or operators like + and *. Operands are leaf nodes;
operator nodes contain references to their operands. (All of these
operators are binary, meaning they have exactly two operands.)

We can build this tree like this:

>>> tree = Tree('+', Tree(1), Tree('*', Tree(2), Tree(3)))

Looking at the figure, there is no question what the order of
operations is; the multiplication happens first in order to compute
the second operand of the addition.

Expression trees have many uses. The example in this chapter uses
trees to translate expressions to postfix, prefix, and infix.
Similar trees are used inside compilers to parse, optimize, and
translate programs.

20.4 Tree traversal

In other words, to print a tree, first print the contents of
the root, then print the entire left subtree, and then print the
entire right subtree. This way of traversing a tree is called
a preorder, because the contents of the root appear before
the contents of the children.
For the previous example, the output is:

To be fair, we should point out that we have omitted an
important complication. Sometimes when we write an expression
in infix, we have to use parentheses to preserve the order of
operations. So an inorder traversal is not quite sufficient to
generate an infix expression.

Nevertheless, with a few improvements, the expression tree and the
three recursive traversals provide a general way to translate
expressions from one format to another.

As an exercise, modify printTreeInorder so that it
puts parentheses around every operator and pair of operands.
Is the output correct and unambiguous? Are the parentheses
always necessary?

If we do an inorder traversal and keep track of what level
in the tree we are on, we can generate a graphical representation
of a tree:

The parameter level keeps track of where we are in the
tree. By default, it is initially 0. Each time we make a
recursive call, we pass level+1 because the child's level
is always one greater than the parent's. Each item is indented by
two spaces per level. The result for the example tree is:

>>> printTreeIndented(tree)
3
*
2
+
1

If you look at the output sideways, you see a simplified version
of the original figure.

20.5 Building an expression tree

In this section, we parse infix expressions and build the
corresponding expression trees. For example, the expression
(3+7)*9 yields the following tree:

Notice that we have simplified the diagram by leaving
out the names of the attributes.

The parser we will write handles expressions that include numbers,
parentheses, and the operators + and *.
We assume that the input string has already
been tokenized into a Python list. The token list for
(3+7)*9 is:

['(', 3, '+', 7, ')', '*', 9, 'end']

The end token is useful for preventing the parser from
reading past the end of the list.

As an exercise, write a function that takes an expression
string and returns a token list.

The first function we'll write is getToken, which takes a token
list and an expected token as arguments. It compares the expected
token to the first token on the list: if they match, it removes the
token from the list and returns true; otherwise, it returns false:

Assuming that getNumber succeeds and returns a singleton tree,
we assign the first operand to a.
If the next character is *, we get the second number
and build an expression tree with a, b,
and the operator.

If the next character is anything else, then we just return
the leaf node with a. Here are two examples:

In other words, a product can be either a singleton or a tree with
* at the root, a number on the left, and a product on the right.
This kind of recursive definition should be starting to feel
familiar.

Next we will add the ability to parse sums. Again, we
use a slightly counterintuitive definition of "sum."
For us, a sum can be a tree with + at the root,
a product on the left, and a sum on the right. Or, a sum
can be just a product.

If you are willing to play along with this definition, it has a nice
property: we can represent any expression (without parentheses) as a
sum of products. This property is the basis of our parsing algorithm.

getSum tries to build a tree with a product on the left and a
sum on the right. But if it doesn't find a +, it just builds a
product.

We are almost done, but we still have to handle parentheses.
Anywhere in an expression where there can be a number, there can
also be an entire sum
enclosed in parentheses. We just need to modify getNumber to
handle subexpressions:

The parser handled the parentheses correctly; the addition happens
before the multiplication.

In the final version of the program, it would be a good idea
to give getNumber a name
more descriptive of its new role.

20.6 Handling errors

Throughout the parser, we've been assuming that expressions are
well-formed. For example, when we reach the end of a
subexpression, we assume that the next character is a close
parenthesis. If there is an error and the next character is something
else, we should deal with it.

The raise statement creates an exception; in this
case a ValueError. If the function that called
getNumber, or one of the other functions in the
traceback, handles the exception, then the program
can continue. Otherwise, Python will print an error message
and quit.

As an exercise, find other places in these functions where errors
can occur and add appropriate raise statements.
Test your code with improperly
formed expressions.

20.7 The animal tree

In this section, we develop a small program that uses a tree
to represent a knowledge base.

The program interacts with the user to create a tree of questions
and animal names. Here is a sample run:

Are you thinking of an animal? y
Is it a bird? n
What is the animals name? dog
What question would distinguish a dog from a bird? Can it fly
If the animal were dog the answer would be? n

Are you thinking of an animal? y
Can it fly? n
Is it a dog? n
What is the animals name? cat
What question would distinguish a cat from a dog? Does it bark
If the animal were cat the answer would be? n

Are you thinking of an animal? y
Can it fly? n
Does it bark? y
Is it a dog? y
I rule!

Are you thinking of an animal? n

Here is the tree this dialog builds:

At the beginning of each round, the program starts at the top of the
tree and asks the first question. Depending on the answer, it moves
to the left or right child and continues until it gets to a leaf
node. At that point, it makes a guess. If the guess is not correct,
it asks the user for the name of the new animal and a question that
distinguishes the (bad) guess from the new animal. Then it adds a
node to the tree with the new question and the new animal.

Here is the code:

defanimal():
# start with a singleton
root = Tree("bird")

# loop until the user quits
while True:
print
if not yes("Are you thinking of an animal? "): break